Lines around the circumference of a curved cylinder will converge toward the inside of the curve and expand toward the outside edge. If you have a slinky around the house, bend it around and watch what happens.When you look at the slinky from the side, you will see the lines on the surface curves toward you bend like this ( ). The lines on a surface curving away from you bend like this )(.

Look closely at the lines that describe the curving oak branch at the top of this post. What is the orientation of the lines around curves that bulge toward you? How is this different than the way the lines converge on surfaces that curve away from you? How do ( ) and )( lines suggest curvature on this branch? Understanding these lines is extremely helpful to draw trees with dimension and accuracy.

Click on the first image below to watch an annotated step-by-step slide show.

With branches drawn as sets of parallel lines, there is no cue to suggest that the branches are curving toward or away from you.

By pulling some of the edge lines into the tree trunk, I can show that the curve on the right swings away from the viewer.

By adding interior lines, I can add additional curves and undulations. Note how ( ) like marks bracket the knot hole on the left. This forces that part of the branch toward the viewer. On the top left, the lines on the branch go the other way, )(, suggesting a curve that swings away from you.